Three Decades of Litter Manipulation Distinctly Shifts Soil Organic Matter Composition and Constrains Soil Carbon Sequestration in Temperate Forest Soils

The capacity of forest soils to sequester carbon (C) is susceptible to shifts in the litter quantity and chemistry. Using measurements from the third decade of a long-term Detrital Input and Removal Treatment (DIRT) experiment at Harvard Forest (MA, USA), extending previously published work from 20 years, we examined how litter inputs shape soil organic matter (SOM) chemistry. Elemental analysis, targeted compound analysis, solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, and microbial biomass and community composition measurements were used. Despite doubled litter inputs over 30 years, no net soil C accumulation occurred, and the SOM decomposition stage was similar to the control, suggesting continuous microbial processing of added inputs. The exclusion of litter, roots, or both led to lower soil C and more advanced SOM decomposition in mineral soils. Shifts in microbial community composition, particularly an increase in Gram (+) to Gram (-) bacteria under exclusion treatments, point to microbial reorganization in response to altered substrate availability. This long-term study underscores the limited potential for long-term soil C sequestration due to sustained microbial decomposition and the role of continuous plant inputs in shaping SOM chemisty under changing detrital regimes in a temperate forest.